For October expo I was thinking of possibly trying to get a GLaDOS cosplay together from the games Portal and Portal 2, however after looking at other peoples costumes I realised I may need some advice and help on what I want to do

For this I want to make a kind of head/eye piece for her to go over my left eye similar to what other cosplayers have done in the past.

There is no fixed way to be GLaDOS so it's not hard to be recognizable. As long as you have a mostly white costume with black being the main other colour used correctly, then you've got yourself a good GLaDOS